Variable lift valves

ABSTRACT

A mechanism is described for achieving variable lift of a valve of an internal combustion engine having an overhead camshaft disposed above the valves. The mechanism comprises two levers both pivoted about fixed fulcrums and a coupling member which is movable along overlapping arms of the two levers to vary the mechanical advantage and thereby vary the valve lift in dependence upon the position of the coupling member.

The present invention relates to an arrangement for varying the degreeof valve lift in an internal combustion engine and in particular to anarrangement which permits such variation without at the same timealtering the phase of opening of the valve.

The volume of the intake charge of any cylinder varies with theoperating conditions and if a fixed degree of valve lift is used underall operating conditions, as is the case in conventional engines, thenthe velocity of the charge through the skirt area and the resultantturbulence in the charge will vary over the engine operating range. Itis clearly desirable, therefore, to be able to vary the valve lift so asto enable the air flow conditions to be optimised over the fulloperating range and in particular to permit higher air velocities underlow speed part load conditions.

The prior art discloses arrangements which permit the geometry of theintake port to be varied with the same aim in mind but these are lesseffective in the control of the air velocities since it is at the skirtarea that the velocity is of importance and the closer the controlmechanism is to this region, the more effective it will prove.

Variable lift valves have been proposed (see for example GB No. 1201872,GB No. 1201558 and GB No. 682628) in which the position of a fulcrum ofa rocker acting between a push rod and a valve can be varied to alterthe mechanical advantage of the rocker and thereby alter the valve lift.Such arrangement have the disadvantage that they cannot be used in aoverhead camshaft engine in which the camshaft is mounted in line withand above the valve stems.

According to the present invention, there is provided a mechanism forachieving variable lift of a valve of an internal combustion enginehaving a fixed profile overhead camshaft, characterised by a leverarrangement interposed between the camshaft and the valve, which leverarrangement includes a first fixed fulcrum lever acted upon by thecamshaft, a second fixed fulcrum lever arranged to actuate the valve anda coupling member movable relative to the fulcrums of the two levers fortransmitting the movement of the first lever to the second lever.

Each of the two levers may be constructed as a single arm lever but,depending upon the geometry of the cylinder head, one or both of thelevers may alternatively be two armed levers. It is important in allcases that one arm of the first lever should overlie an arm of thesecond lever so that movement of one can be directly transmitted to theother.

The travel of the second lever will vary with the position of thecoupling member and in some cases, depending upon the lift height andthe separation of the levers, the lower lever may overlap the positionof the upper lever in a horizontal plane. Collision between the leverscan be avoided by offsetting one of the levers to one side in thehorizontal plane but this step alone would cause a bending moment to beapplied to the coupling between the levers. It is therefore preferred inthis case for the first lever to be bifurcated and to straddle thesecond lever. This bifurcation of the first lever is also effective toprevent direct collision between the first lever and the valve springretainer plate.

The coupling member between the levers may comprise a roller movablealong the length of the overlapping lever arms. Such a roller may besupported on a sheet metal cage and movement of the roller can beeffected by a rack and pinion mechanism in which the rack is constitutedby or supported on the cage while the pinion can be formed on or drivenby the shaft on which one of the levers is journalled.

As an alternative, the coupling member may consist of a roller carriedby a further lever pivotably mounted on the outer end of an armprojecting from the shaft on which one of the levers is journalled. Asthe latter shaft is turned, the further levers for all the cylinders aremoved in unison to vary the positions of their respective rollersrelative to the first and second levers of the valves.

It is an important advantage of the invention that the point of contactof the second lever with the valve does not vary with the valve lift andconsequently a conventional hydraulic tappet may be incorporated in thesecond levers so that free play between the cam and valve is taken up atall times. In this case, oil pressure to the hydraulic tappets may betransmitted along the support shaft and the second levers.

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a prior art arrangement forachieving variable valve lift,

FIG. 2 is a plan view of an embodiment of the present invention, and

FIG. 3 shows a similar view to that of FIG. 1 of the embodiment of theinvention shown in FIG. 2.

FIG. 4 shows a second embodiment according to the present invention.

In FIG. 1, there is shown a camshaft 10 acting on a valve 18 against theforce of the valve spring 20 by way of a train comprising a cam follower12 and a rocker lever 14 pivotable about a fulcrum 16.

Whereas the fulcrum of the rocker lever of conventional engine isusually fixed, in the arrangement shown in FIG. 1, the position of thefulcrum 16 is adjustable to vary the mechanical advantage of the leverand thereby adjust the maximum lift of the valve 18 even though the cam10 has a fixed profile. To this end, the mechanism of the fulcrum 16comprises two blocks 24 and 26 joined to one another by a pivot pin 28.The block 26 can slide along a track 22 while the block 24 can slidealong the length of the rocker lever 14, which is also formed with asuitable track for guiding the block 24. Though movable in the plane ofthe drawing, because of the tracks, the fulcrum mechanism preventsmovement of the rocker lever 14 out of the plane of the drawing.

While a mechanism as shown in FIG. 1 can vary valve lift in an engine inwhich the cam 10 is mounted to one side of the valves 18, it cannot beused in overhead camshaft engines in which the camshaft is arrangedabove the stems of the valves.

The embodiment of the present invention illustrated in FIGS. 2 and 3overcomes this problem and comprises two interacting levers 50 and 52interposed between the camshaft 10, and the valve 18'. The lever 50 is afixed fulcrum lever pivotable about a shaft 54 and acted upon directlyby the camshaft 10, The lever 52 has two arms 52a and 52b which move inunison about a shaft 56. The lever arm 52a is disposed below the lever50 and is moved by the latter through a coupling member 58 which isshown as being a pin. The resulting movement of the lever arm 52b causesthe valve 18' to be opened against the action of the valve spring (notshown).

The mechanical advantage of the mechanism comprising the levers 50 and52 depends upon the position of the coupling member 58. To permit thisposition to be adjusted, the coupling member 58 is mounted on a link arm60 assembly formed of two plates straddling the coupling member 58 andpivotable about the axis of a pin 62 passing through an adjusting arm 64projecting radially from the shaft 56 and mounted for rotation with theshaft 56.

If the shaft 56 is rotated, the adjusting arm 64 moves the position ofthe pin 62 which now moves the link arm 60 to reposition the couplingmember 58 between the two levers 50 and 52a. The closer the couplingmember 58 to the shaft 54, the less the lift of the valve 18,

In any one position of the coupling member 58, the levers 50 and 52awill both rotate relative to the coupling member 58 during the openingof the valve and only line contact can be maintained. For this reason,the coupling member has been shown as a pin rather than a flat block.However, such line contact may cause severe wear and it is possible tosubstitute for the pin a two part block which incorporates a bearingsurface, in a manner analogous to the block shown in FIG. 1.

The valve springs 20 are relatively stiff and one or other of thesprings of an engine is compressed at any angle of the camshaft 10. Thusif movement of all the coupling members 58 is effected simultaneously,then the force required must exceed the resistant of the valve springs.However, there are times when each coupling member associated with oneof cylinder can be moved with minimal effort, namely when the valve isfully closed. To take advantage of this, one may stagger the movement ofthe individual coupling members so that each member 58 fulcrum is movedwhen it presents a minimum resistance.

For example, the coupling member 58 may be moved by way of a springbiased lost motion mechanism arranged between the adjusting arm 64 andthe shaft 56. The spring in the lost motion mechanism is sufficientlystrong to move the coupling member 58 when the valve is closed but nototherwise. If the coupling members 58 of all the cylinders are to beadjusted, then the shaft 56 is rotated to compress the springs of allthe lost motion mechanisms. However, the individual link arms 60 willnot move and displace their coupling members 58 until such time as theassociated valve is closed and the coupling member 58 is not compressedbetween the two levers 50 and 52.

If it is desired to incorporate a hydraulic tappet then this may beplaced between the tip of the valve 18, and the lever arm 52b. Oil forthe hydraulic tappet can be supplied along the pivot shaft 56 and thelever arms 52b.

In the embodiment of FIG. 4, lever 52 has a single arm, 52a, which actsdirectly upon the tip of the valve 18'.

The preferred embodiment of the invention can thus be seen to permitvalve lift to be varied in an engine with an overhead camshaft arrangedabove the valves and to offer the additional advantage of enablingadjustment by a particularly simple and effective control system. Theeffort required for movement of the control system can be minimal thisbeing important in order to avoid power losses.

I claim:
 1. A mechanism for achieving variable lift of a valve of aninternal combustion engine having a fixed profile overhead camshaft,characterised by a lever arrangement interposed between the camshaft andthe valve, which lever arrangement includes a first pivotably fixedfulcrum lever (50) acted upon by the camshaft (10'), a second pivotablyfixed fulcrum lever (52) arranged to actuate the valve (18'), and acoupling member (58) movable relative to respective fulcrums of the twolevers (50,52) for transmitting the movement of the first lever (50) tothe second lever (52), wherein the coupling member comprises a pin (58)carried by a further lever (60) pivotably mounted on an outer end of anarm (64) projecting from a shaft (56) on which one of the first andsecond levers (50,52) is journalled.
 2. A mechanism as claimed in claim1, wherein each of the two levers (50,52) is constructed as a single armlever.
 3. A mechanism as claimed in claim 1, wherein at least one of thelevers (50,52) is a two armed lever.
 4. A mechanism as claimed in claim1, wherein the coupling member (58) is positioned between overlappingportion of the levers (50,52) and comprises a pin movable (58) along thelength of the overlapping lever arms (50,52a).